1. Field of the Invention
The present invention relates in general to liquid crystal display devices and, in more particular, to liquid crystal display devices of the so-called partial transmission type active matrix scheme for performing display operations of the transmission type and reflection type.
2. Description of the Related Art
Active-matrix liquid crystal display devices are typically arranged so that gate signal lines extending in an xe2x80x9cxxe2x80x9d direction and being parallel-provided in a xe2x80x9cyxe2x80x9d direction and drain signal lines extending in the y direction and parallel-provided in the x direction are formed on a liquid crystal side surface of one substrate of substrates that are disposed to oppose each other with a layer of liquid crystal material interposed between them, wherein a region being surrounded by these signals is adapted for use as a picture element or xe2x80x9cpixelxe2x80x9d region.
And, formed in this pixel region are a thin-film transistor which is rendered operative by supplement of a scanning signal from a gate signal line disposed along one side of it and a pixel electrode that is expected to receive an image signal supplied thereto from a drain signal line as disposed on one side through this thin-film transistor.
This pixel electrode is designed to permit creation of an electric field between it and an opposite or xe2x80x9ccounterxe2x80x9d electrode that is formed on a liquid crystal side surface of the other substrate disposed with the liquid crystal layer interposed therebetween, thereby to control the liquid crystal""s optical transmittance or transmissivity.
And, with such an arrangement, liquid crystal display devices of the so-called partial transmission type for performing display of the transmission type and reflection type include a prior known device which is designed so that the pixel electrode is made up from a conductive reflection film and transparent conductive film that are electrically connected together.
More specifically, in the case of using as the transmission type while letting a back-light unit repeat turn-on and -off operations to exhibit blinking, an observer is able to observe the light from such backlight through the transparent conductive film and the layer of optical transmissivity-controlled liquid crystals; alternatively, in the case of using as the reflection type while letting the backlight turn off, he or she can observe external light as reflected off by the conductive reflection film via the optical transmissivity-controlled liquid crystals.
The crystal display device arranged in the way stated above has the following objects to be solved regarding the efficiency of light ray utilization say, light utilization efficiency. To be brief, it has been pointed out that although a specific transparent conductive film with its optical transmissivity of more than 90 percent (%) such as ITO is used for a light pass-through portion while employing a thick Al at a reflective portion for establishment of the surface reflectivity of approximately 80%, part of the pixel region is used as a transmission section whereas the remaining part is for use as a reflection section so that the performance of each is in the xe2x80x9chalf-donexe2x80x9d state.
Note here that Japanese Patent Laid-Open No. 281972/1999 discloses therein a technique for causing light rays from the backlight to pass through a high polymer molecule resin film (used as a protective film) with convexo-concave surface irregularities on the liquid crystal side surface of a transparent substrate as disposed to oppose it while letting reflected light as formed on its upper surface reflect again at the backlight to thereby utilize this as transmitted light.
However, according to experimentation by the inventors as named herein, it has been affirmed that this technique is not sufficient in actual re-reflection efficiency because of the fact that the polymer film""s light absorption coefficient is greater resulting in the light penetrating this film a couple of times.
In addition, as in a pixel structure indicated in the above-identified Japanese patent No. 281972/1999, in the case of employing an ITO as its transparent electrode and then fabricating a polymer resin film with irregular surface configuration and a reflection electrode made of Al at later process steps, it will become difficult to arrange the reflection electrode by use of a single-layer of Al.
This can be said because a developing solution at photo-resist process steps for pattern formation behaves to penetrate into an underlying layer(s) to finally touch the ITO, resulting in unwanted occurrence of electrolytic corrosion due to the presence of a work function difference between the ITO and Al.
Due to this, as shown for example in Japanese Patent Laid-Open No. 231993/1999, certain one is known which is arranged so that the reflection electrode is of a multilayer structure with its underlying layer formed of a high-melting-point metal layer such as Mo or the like and also with its overlying layer made of Al.
However, in the case of fabrication of the reflection electrode in this way, there is a penalty which follows: the resultant efficiency for utilizing the light from the backlight as the required transmission light while forcing it to experience reflection again is hardly sufficient because of the fact that Mo is lower in reflectivity than Al.
Alternatively, with regard to ones of the active matrix type comprising a thin-film transistor at each pixel as manufacturable through several processes requiring increased precision, these ones suffer from deficiency in production yields; in particular, they are strictly required to absolutely avoid risks of electrical shortcircuiting between neighboring gate signal lines or adjacent drain signal lines.
The present invention has been made in view of the above technical background, and its primary object is to provide a liquid crystal display device capable of improving the optical transmissivity without reducing the optical reflectivity thereof.
It is another object of this invention to provide a liquid crystal display device capable of avoiding electrical shortcircuiting between neighboring signal lines.
A brief summary of a representative one of those inventive principles of the invention as disclosed herein will be set forth below.
In summary, the liquid crystal display device in accordance with the instant invention is featured in that light from a back-light unit is irradiated from the side of one substrate of respective substrates disposed to oppose each other with a layer of liquid crystal material being interposed therebetween, that a region on a liquid crystal side surface of the one substrate as surrounded by neighboring gate signal lines and neighboring drain signal lines is adapted for use as a pixel region, that a pixel electrode is formed within this pixel region, to which electrode an image signal from one drain signal line is supplied via a thin-film transistor as rendered operative by supplement of a scan signal from one gate signal line, that this pixel electrode is constituted from a conductive reflection film and a transparent conductive film being electrically connected together, that a light reuse film is formed beneath the conductive reflection film through the conductive reflection film and a dielectric film, and that this light reuse film is electrically isolated from either the gate signal lines or the drain signal lines.
The liquid crystal display device thus arranged is such that fabrication of the light reuse film in particular permits light from the backlight to reflect onto this light reuse film and further reflect at the surface of the backlight to thereby enable it to pass through the transparent conductive film which is part of the pixel electrode.
Additionally, since this light reuse film is laid out beneath the conductive reflection film with a dielectric film interposed therebetween, there are no inconveniences as to spaces at any remaining members, which will become the one that utilizes so-called dead spaces; thus it is possible to enlarge the area thereof, which in turn makes it possible to achieve efficient utilization of light rays concerned.
In addition, as this light reuse film is formed to have its unique effects unlike the one that requires other members to also have light reutilization capabilities, it becomes possible to offer its unique choiceability of material or else due to the reason for improvement in reflection efficiency or the like.
Furthermore, as this light reuse film is electrically isolated from signal lines, any capacitive coupling will no longer take place between the film and any pixel electrode overlapping it through a dielectric film; thus it is possible to avoid the problem as to unwanted potential variation or deviation at such pixel electrode.
Moreover, as this light reuse film is electrically isolated from signal lines, it is also possible to avoid the risk of electrical shortcircuiting between neighboring ones of the signal lines which are adjacent each other with this light reuse film laid between them.